In many technical applications components are assembled with threaded fasteners. For example, where it is desired to assemble a first component to a second component, tapped holes are formed in one component while corresponding clearance holes are formed in the second component. The two components are positioned with respect to each other so that the clearance holes are aligned with the tapped holes. Threaded fasteners, such as screws or bolts or the like, are then used to secure the components together.
Usually, the machine tools that are used to form such tapped and clearance holes can easily provide the required accuracy in the positioning of such holes for proper alignment and fit of the components to be assembled without the use of additional gauging and measuring equipment. This is especially true where high precision alignment between the components is not required. However, where high precision alignment of components is required, gauging devices may be needed to facilitate and increase the accuracy of measuring the locations of hole positions (center axes, for example) with respect to each other and/or with respect to other reference points. Often, such high precision is required for alignment of optical systems such as guidance and navigation systems.
One technique that can be used to indicate the centerline of a hole includes the use of a gauging device. One such gauging device for indicating the centerline of a tapped hole (or a bored hole) is shown in U.S. Pat. No. 3,026,621 to Papps et al. The hole position gauging device of Papps et al. has a threaded section adapted to screw into the hole to be indicated and a concentric cylindrical stem that extends above the hole. The tolerances for the concentricity of the stem relative to the threaded section and the diameter of the stem are known so that the stem can indicate, within a known tolerance, the centerline of the hole in which the gauging device is installed. For example, the stem can be contacted with a caliper or other position indicator in order to determine the position of the centerline of the tapped hole relative to another tapped hole, a machine tool, reference structure, or in a predetermined frame of reference.
Another device for locating the centerline of a tapped hole is shown in U.S. Pat. No. 5,179,788 to Jadach. The gauging device of Jadach is similar to the gauging device of Papps et al. except that the Jadach device uses a counterbore instead of a stem as an indicating feature. The counterbore allows for faster and easier measurement with programmable touch probe coordinate measuring machines in that fewer moves by the probe are needed to contact plural points on the inside surface of the counterbore as compared to using a stem. This is because plural points of the counterbore surface can be contacted by moving the probe along a linear path within the counterbore and the probe does not need to be moved over or around the stem.
One problem with the gauging devices of Papps et al. and Jadach is that a typical threaded fastener and the tapped hole into which it fits are intentionally designed to have some clearance or play between them in order to allow them to be separable. That is, they are not a perfect fit with each other in order to avoid interference that could cause them to become permanently locked together. Because of this, the threaded section of the devices of Papps and Jadach inherently has some play when installed in a threaded hole as the devices are designed to be reusable. This play can introduce measurement errors because the gauge is not locked in place with respect to the tapped hole and reduces the overall precision of the gauging device.
Another problem with the gauging devices of Papps et al. and Jadach is that they are designed to seat on a flat surface. As such, these devices are generally unsuitable for use with uneven or irregular surfaces. This is because any deviation in the perpendicularity of the centerline of the tapped hole with respect to the surface can introduce measurement errors. Conventional hole gauges, similar to the gauges of Papps et al. and Jadach, are available for use with uneven or irregular surfaces. These hole gauges have a threaded portion that is tapered. When such a tapered threaded portion is threaded into a tapped hole in a workpiece, the taper causes the threads of the threaded portion to interfere with the threads of the tapped hole thereby locking the gauge in the threaded hole. However, because the interference occurs near the surface of the workpiece and at the top of the tapped hole as a result of the tapered structure, these gauges are susceptible to being canted after being installed especially if the gauge is unintentionally bumped or jarred.